U.S. Pat. No. 3,647,992 discloses a circuit arrangement for two-wire/four-wire conversion in a DMT system, which is connected to a digital reception path, a digital transmission path and also an analog transmission/reception path and which has nonlinear echo cancellation in the time domain and also has a device for adaptation of the nonlinear echo cancellation in the frequency domain.
U.S. Pat. No. 5,778,055 discloses nonlinear echo cancellation in connection with an analog telephone system.
U.S. Pat. No. 5,317,596 describes a method and a device for echo cancellation in the case of a signal generated by discrete multitone modulation. As is known, full-duplex data transmission is the simultaneous transmission of data in two opposite directions with partly overlapping frequency bands. In this case, an echo occurs as a result of a signal to be transmitted crossing into the receiver situated at the same location, as a result of which a signal received by the remote end is superposed and corrupted. By way of example, an echo occurs in a telephone network if the signal transmitted by a user passes through the hybrid circuit into the receiver of the user. This hybrid circuit may also be referred to as an echo channel. Such an echo channel can be modeled by a finite number of parameters. Therefore, an echo canceler first estimates the parameters and then linearly convolves the estimation with the transmitted signal, as a result of which the echo is emulated. The emulated echo signal thus obtained is then subtracted from the received signal, whereby the pure signal is produced in the ideal case.
Echo principally occurs owing to inexactly matched impedances at the hybrid connectors. Since the impedances of the transmission lines are time-dependent and line-dependent, the echo canceler must be adaptive. Furthermore, it is possible to carry out echo cancellation both in the time domain and in the frequency domain of a signal.
In multicarrier modulation, the data to be transmitted are transmitted by the binary digital data that are to be transmitted first being arranged in sub-blocks. These sub-blocks are then combined to form blocks or fixed length which are then in each case modulated onto a carrier and transmitted. Discrete multi-tone modulation is a form of multicarrier modulation which is used in digital signal processing, an IFFT/FFT pair being used as modulation/demodulation vector.
In the US patent specification mentioned, the echo cancellation is performed both in the time domain and in the frequency domain of a signal. What is disadvantageous is that only linear echo cancellation is performed, so that complete echo cancellation is not achieved.
Furthermore, methods are known in which the cancellation of the nonlinear echo signal is effected in the time domain and the attenuation of the linear echo signal is preferably effected in the frequency domain. This gives rise to difficulties in the adaptation due to slow transient recovery and convergence problems.
Furthermore, WO 98/32241 describes a circuit arrangement for two-wire/four-wire conversion, in which digital signals of a digital reception path are converted and coupled via a hybrid onto an analog transmission/reception path and analog signals of the analog transmission/reception path are digitized and coupled onto a digital transmission path, echo suppression being connected between the digital transmission and reception paths. In this case, an echo estimation filter is used to adaptively approximate the behavior of the hybrid circuit and of the analog transmission/reception path. In this case, the echo cancellation is effected both in the frequency domain and in the time domain of the signals, which causes difficulties in the adaptation.